The application of brief, high intensity electrical pulses has long been known to cause a transient state of permeability in the membranes of cells. It has been demonstrated in vitro that the intracellular concentration of normally impermeant substances, such as dyes (Mir et al. Exp. Cell Research 175: 15-25, (1988)), genes ((Chang, D. et al. in "Guide to Electroporation and Electrofusion" D. Chang ed., Academic Press, San Diego, pp. 1-6 (1992)), drugs (Poddevin et al., Biochem Pharmacol 42 Suppl: S67-S75 (1991)), and proteins (Mouneimne et al., Biochim et Biophys Acta 1027:53-58 (1990)) can be dramatically increased while cells exhibit this transient permeability. This process, known as electroporation, has also been applied in vivo to increase the permeability of cells in tissue.
Many studies have reported a substantial improvement in the cytotoxicity of certain membrane-limited chemotherapeutic drugs when used in conjunction with electroporation. Animal studies (Mir et al., Eur J Cancer 27:68-72 (1991), Belehradek et al., Eur J Cancer 27:73-76 (1991), Heller et al., Bioelectrochem Bioenerg 36:83-87 (1995), Jaroszeski et al., Biochim et Biophys Acta 1334:15-18 (1997)) and early human trials (Domenge et al., Cancer 77:956-963 (1996), Heller et al., Cancer 77:964-966 (1996)) have indicated that electroporation therapy shows great promise as a treatment for solid tumors because the chemotherapeutic drugs delivered by this technique can be made very effective while minimizing side effects.
While the initial development of electroporation technology has been in the area of drug delivery, another application of electroporation therapy currently being studied involves the use of gene therapy. Although gene therapies are currently being developed for the treatment of many diseases, including cancer, diabetes, heart disease, and arthritis, a safe and reliable technique for their delivery has yet to be developed for clinical use. Several studies (Heller et al., FEBS Letters 389:225-228 (1996), Rols et al., Nature Biotechnology 16:168-171 (1998), Harimoto et al., Brit. J. Urology 81:870-874 (1998)) demonstrate that transfection and expression of marker genes, such as luciferase and .beta.-galactosidase, can be improved in vivo by the application of electrical pulses to the tissue of a targeted area. These results suggest that electroporation may provide a feasible method for the transfection of genetic material into living cells in tissue.
The use of electroporation therapy for the transmembrane delivery of therapeutic substances is dependent on achieving two necessary and sufficient conditions in the region to be treated: (I) Adequate concentration of therapeutic substance must be present in the extracellular space, and (II) Threshold level electrical fields must be generated throughout the target tissue. While a significant amount of research has been performed demonstrating the utility of electroporation in the treatment of various animal and human tumor models (Heller et al. (1995), Hofmann et al., IEEE Eng Med and Biol, 124-132 (November/December 1996), Jaroszeski et al. (1997)), there is limited understanding regarding the best methods for the clinical application of electroporation therapy.
In the field of cancer treatment, delivery of therapeutic substances is made more difficult by the anatomical characteristics of solid tumors such as nonuniform vasculature and high interstitial pressure. These properties make it difficult to achieve uniform, high concentrations of therapeutic substances within the tumor (Jain, R., Scientific American 271(1):58-65 (1994)). The tortuous, nonuniform vasculature prevents blood borne substances from reaching all parts of the tumor. Due to high interstitial pressures, maintaining the necessary concentrations of drug within the tumor is also difficult, because this pressure gradient causes substances to be forced back into the vasculature or carried by convection to the exterior of the tumor. The nature of current chemotherapeutic drugs also limits their effectiveness. While administration of drugs into the vasculature provides excellent distribution, systemic dosages of therapeutic substances are often limited by their toxic side effects. Therefore, a higher concentration of therapeutic substance cannot be achieved simply by increasing the systemic dosage, without serious risk of harm to the patient.
Given the problematic nature of delivering high levels of therapeutic substance to solid tumors, electroporation therapy seems well suited to the treatment of these cancers. However, methods must be employed to ensure that sufficient levels of therapeutic substance are present in the interstitial space when the permeabilizing pulses are delivered. Because membrane permeability occurs as a result of exposing a cell to threshold level electric field strengths, an effective electroporation therapy is dependent on propagating these fields throughout a target region of tissue and allowing sufficient concentrations of the desired substances to accumulate intracellularly.
Thus, it is considered desirable to provide a means for increasing the amount of therapeutic substance which accumulates in the cells of electroporated tissue.